While Nd—Fe—B sintered magnets, referred to as Nd magnets, hereinafter, are regarded as the functional material necessary for energy saving and performance improvement, their application range and production volume are expanding every year. Since the automotive application assumes service in a hot environment, the Nd magnets incorporated in driving motors and power steering motors in hybrid vehicles and electric vehicles must have high coercivity as well as high remanence. The Nd magnets, however, tend to experience a substantial drop of coercivity at elevated temperature. Then the coercivity at room temperature must be preset fully high in order to insure an acceptable coercivity at service temperature.
As the means for increasing the coercivity of Nd magnets, it is effective to substitute Dy or Tb for part of Nd in Nd2Fe14B compound as main phase. For these elements, there are short reserves, the mining areas amenable to commercial operation are limited, and geopolitical risks are involved. These factors indicate the risk that the price is unstable or largely fluctuates. Under the circumstances, in order that R—Fe—B magnets adapted for high-temperature service find a wider market, a new approach or magnet composition capable of increasing coercivity while minimizing the content of Dy and Tb is needed.
From this standpoint, several methods are already proposed. Patent Document 1 discloses an R—Fe—B base sintered magnet consisting essentially of 12-17 at % of R (wherein R stands for at least two of yttrium and rare earth elements and essentially contains Nd and Pr), 0.1-3 at % of Si, 5-5.9 at % of boron, 0-10 at % of Co, and the balance of Fe (with the proviso that up to 3 at % of Fe may be substituted by at least one element selected from among Al, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, In, Sn, Sb, Hf, Ta, W, Pt, Au, Hg, Pb, and Bi), containing an intermetallic compound R2(Fe,(Co),Si)14B as main phase, and exhibiting a coercivity of at least 10 kOe. Further, the magnet is free of a boron-rich phase and contains at least 1 vol % based on the entire magnet of an R—Fe(Co)—Si grain boundary phase consisting essentially of 25-35 at % of R, 2-8 at % of Si, up to 8 at % of Co, and the balance of Fe. After sintering or heat treatment following sintering, the sintered magnet is cooled at a rate of 0.1 to 5° C./min at least in a temperature range from 700° C. to 500° C., or cooled in multiple stages including holding at a certain temperature for at least 30 minutes on the way of cooling, for thereby generating the R—Fe(Co)—Si grain boundary phase.
Patent Document 2 discloses a Nd—Fe—B alloy with a low boron content. A sintered magnet is prepared by sintering the alloy and cooling the sintered product below 300° C. The step of cooling down to 800° C. is at an average cooling rate ΔT1/Δt1<5 K/min.
Patent Document 3 discloses an R-T-B magnet comprising a main phase of R2Fe14B and some grain boundary phases. One of the grain boundary phases is an R-rich phase containing more R than the main phase, and another is a transition metal-rich phase having a lower rare earth concentration and a higher transition metal concentration than the main phase. The R-T-B rare earth sintered magnet is prepared by sintering at 800 to 1,200° C. and heat treatment at 400 to 800° C.
Patent Document 4 discloses an R-T-B rare earth sintered magnet comprising a grain boundary phase containing an R-rich phase having a total atomic concentration of rare earth elements of at least 70 at % and a ferromagnetic transition metal-rich phase having a total atomic concentration of rare earth elements of 25 to 35 at %, wherein an area proportion of the transition metal-rich phase is at least 40% of the grain boundary phase. The sintered magnet is prepared by shaping an alloy material into a compact, sintering the compact at 800 to 1,200° C., and a plurality of heat treatments, i.e., first heat treatment of heating at a temperature of 650 to 900° C., cooling to 200° C. or below, and second heat treatment of heating at 450 to 600° C.
Patent Document 5 discloses an R-T-B rare earth sintered magnet comprising a main phase of R2Fe14B and a grain boundary phase containing more R than the main phase, wherein the main phase of R2Fe14B has an axis of easy magnetization parallel to c-axis, crystal grains of the main phase are of elliptic shape elongated in a direction perpendicular to the c-axis, and the grain boundary phase contains an R-rich phase having a total atomic concentration of rare earth elements of at least 70 at % and a transition metal-rich phase having a total atomic concentration of rare earth elements of 25 to 35 at %. Also described are sintering at 800 to 1,200° C. and subsequent heat treatment at 400 to 800° C. in an argon atmosphere.
Patent Document 6 discloses a rare earth magnet comprising a main phase of R2T14B crystal grains and an intergranular grain boundary phase between two adjacent R2T14B main phase crystal grains, wherein the intergranular grain boundary phase has a thickness of 5 nm to 500 nm and is composed of a phase having different magnetism from ferromagnetism. The intergranular grain boundary phase is formed of a compound which contains element T, but does not become ferromagnetic. Thus, the intergranular grain boundary phase contains a transition metal element and element M such as Al, Ge, Si, Sn or Ga. By further adding Cu to the rare earth magnet, a crystalline phase with a La6Co11Ga3-type crystal structure may be evenly and broadly formed as the intergranular grain boundary phase, and a thin R—Cu layer may be formed at the interface between the La6Co11Ga3-type intergranular grain boundary phase and the R2T14B main phase crystal grains. As a result, the interface of the main phase can be passivated, the generation of strain due to a lattice mismatch be suppressed, and reverse magnetic domain-generating nuclei be inhibited. The method of preparing the magnet involves sintering, heat treatment at a temperature of 500 to 900° C., and cooling at a cooling rate of at least 100° C./min, especially at least 300° C./min.
Patent Documents 7 and 8 disclose an R-T-B sintered magnet comprising a main phase of Nd2Fe14B compound and an intergranular grain boundary phase between two main phase grains, with a thickness of 5 to 30 nm, and having a grain boundary triple junction surrounded by three or more main phase grains.